Multilevel converters are used in power electronics to generate a multilevel, “stepped” voltage which has only small harmonics portions even at a low pulse frequency of the power semiconductors which are used, without costly LC filters. The topology and control of the modular multilevel converters, which optionally permit a series connection and parallel connection of their modules are explained to a certain degree in GOETZ, Stefan M.; PETERCHEV, Angel V.; WEYH, Thomas. Modular multilevel converter with series and parallel module connectivity: Topology and control. IEEE Transactions on Power Electronics, 2015, 30th year, No. 1, pp. 203-215, which is incorporated by reference herein.
Multilevel converters of the generic type permit, for example, the conventionally hard-wired battery packs to be divided up structurally into battery modules of the same type in such a way that the electrical connection of the individual parts can be changed dynamically during operation. In particular in the case of large batteries as in electric vehicles or network storage units there is a trend toward such formation of modules. Any electronics present are usually mounted in these modules on an upper or lateral face.
Batteries with multilevel converters which are integrated in such a way or similarly complex power electronics therefore have two heat sources: on the one hand the battery cells heat up owing to their considerable internal resistance both when charging and discharging; on the other hand, the power electronics themselves conduct large currents through electronics components which generate additional conduction losses and switching losses. This additional heat source in the electronics is not provided in conventional battery concepts.
The battery cooling system therefore determines to a high degree the power of an electric vehicle. In the case of electric cars according to the prior art, a costly liquid cooling system is therefore sometimes used, which cools the batteries. DE102013020961, which is incorporated by reference herein, describes such a battery system which comprises a power electronics component or a power electronics assembly for switching an electric load on and off, having a coolant circuit between the electronics and the battery. CN106130408, which is incorporated by reference herein, also discloses vehicle power electronics which are coupled to the battery for the purpose of cooling. A common coolant circuit for vehicle battery and power electronics is also mentioned in DE102011012723, DE102014009772, US2012104843, US2012152186, US2016226421 and US2016318370, all of which are incorporated by reference herein.